This invention relates to an explosive carrier for the perforation of downhole casing and the penetration of earth formation therefrom during oil and gas production operations.
In oil and gas operations, perforating through casing using a perforating gun is probably the most important of all completion jobs in cased holes. After a casing is properly placed in a drilled hole, a charge carrier carrying explosive charges is lowered downhole. Charges are fired to effectuate perforations through the steel casing and into the earth formation therefrom, thereby providing communication between the wellbore and the desired producing zones.
In conventional charge carriers, the explosive charges are arranged in a spiral configuration. To minimize interference between the charges, the explosive charges in single-spiral conventional carriers are spaced at 60.degree. phasing and at a vertical distance of about 2 inches. Such a conventional configuration results in a shot density of 6 shouts per foot. Because of the limited spacing, there is a certain amount of interference between the firing of shots. Due to the pressure wave generated by neighboring shots and by the detonator itself, the hole size is often significantly smaller than that which could be achieved if no such interference existed.
With these charge carriers, in order to achieve a desired flow rate, the same cased hole often has to be shot twice. The charge carrier is first lowered into the wellbore, and shots are fired. The carrier is then pulled back to the surface and reloaded with charges. The charge carrier is then lowered again into the wellbore and refired. Safety is a serious concern in such multiple trip operations due to the use of explosives. Some of the explosive charges may not have properly detonated and could explode at the surface and cause serious injury.
Because the charge carrier must be lowered twice, the possibility that the carrier may get stuck in the pipe and require a laborious fishing job is doubled. Multiple trips also consume significant rig time, which could be very expensive, especially during offshore operations. If the charge carrier is not properly positioned in the second run, it could end up shooting the same hole twice. A multiple shooting also carries the risk of splitting the casing when two shots are fired together.
Increased shot density has been achieved in conventional charge carriers by arranging the charges in three spirals, 120.degree. out of phase with each other. These charge carriers can achieve 12 shots per foot, but still retain several of the disadvantages of single-spiral charge carriers. First, the shots of the three spirals are clustered so that each shot is grouped at the same axial position as two others, even though the shots are 120.degree. out of phase circumferentially. This clustering of three shots at each level leaves the layers of earth between each cluster unperforated. Since it is the nature of subterranean hydrocarbons to flow along the layers of bearing strata, conventional charge carriers leave some producing strata unperforated.
Multi-spiral conventional carriers also retain the single-spiral carriers' problem of interference between shots and the resulting decrease in hole size. Multi-spiral conventional carriers also detonate the charges in the same manner as single-spiral conventional carriers, and thus suffer from the same type of interference found in the single-spiral carriers. The problem is aggravated by the detonation of the charges in a single spiral, followed by the next spiral, and so on.
Finally, multi-spiral conventional carriers typically use 120.degree. phasing between spirals and 60.degree. phasing between individual charges in a given spiral. This configuration results in reduced casing strength, because it places multiple perforations on each plane of failure (which runs perpendicular to the application of load on the casing). The casing, thus weakened, is subject to a much greater risk of crushing and the well therefore bears a much greater risk of costly rework. The present invention distributes the perforations around the wellbore so that the number of perforations on each plane of failure is reduced, thereby retaining most of the strength of the original unperforated well casing.